T Cell Receptors (TCR) are primary effectors of the immune system that have unique advantages as a platform for developing therapeutics. While antibody therapeutics are limited to recognition of pathogens in the blood and extracellular spaces or to protein targets on the cell surface, T cell receptors can recognize antigens displayed with MHC molecules on the surfaces of cells (including antigens derived from intracellular proteins). Depending on the subtype of T cells that recognize displayed antigen and become activated, T cell receptors and T cells harboring T cell receptors can participate in controlling various immune responses. For instance, T cells are involved in regulation of the humoral immune response through induction of differentiation of B cells into antibody producing cells. In addition, activated T cells act to initiate cell-mediated immune responses. Thus, T cell receptors can recognize additional targets not available to antibodies.
T-cells are a subgroup of cells which together with other immune cell types (polymorphonuclear, eosinophils, basophils, mast cells, B-, NK cells) constitute the cellular component of the immune system. Under physiological conditions T-cells function in immune surveillance and in the elimination of foreign antigen. However, under pathological conditions there is compelling evidence that T-cells play a major role in the causation and propagation of disease. In these disorders, breakdown of T-cell immunological tolerance, either central or peripheral is a fundamental process in the causation of autoimmune disease.
The TCR is believed to play an important role in the development and function of the immune system. For example, the TCR has been reported to mediate cell killing, increase B cell proliferation, and impact the development and severity of various disorders including cancer, allergies, viral infections and autoimmune disorders.
It thus would be desirable to provide novel targeting agents based on T cell receptors, as well as methods for producing and using such agents for therapeutic and diagnostic settings. Accordingly, it would be particularly desirable to provide such molecules that would have certain advantages in comparison to prior art complexes based on antibody targeting.
Moreover, it is desirable to use the TCR to target various effector molecules to the disease site where they can provide therapeutic benefit without the side effects associated with system non-targeted activity. One such is IL-15, a member of the four alpha-helix bundle family of lymphokines. IL-15 plays a multifaceted role in development and control of the immune system. More specifically, IL-15 influences the function, development, survival, and proliferation of CD8+ T cells, NK cells, killer T cells, B cells, intestinal intraepithelial lymphocytes (IEL) and antigen-presenting cells (APC). It has been demonstrated that both IL-15−/−, and IL-15Ra−/− transgenic mice lack peripheral NK and killer T cell populations, certain IEL subsets, and most memory phenotype CD8+ T cells, suggesting IL-15 plays role in the development, proliferation or/and survival of these cell types. The IL-15 receptor (R) consists of three polypeptides, the type-specific IL-15R alpha (“IL-15Rα” or “IL-15Ra”), the IL-2/IL-15Rbeta (“IL-2Rβ” or “IL-15Rb”), and the common gamma chain (“γC,” or “gC” which is shared by multiple cytokine receptors).
IL-15 signaling can occur through the heterotrimeric complex of IL-15Rα, IL-2Rβ and γC; through the heterodimeric complex of IL-2Rβ and γC. A novel mechanism of IL-15 action is that of transpresentation in which IL-15 and IL-15Rα are coordinately expressed by antigen-presenting cells (monocytes and dendritic cells), and IL-15 bound to IL-15Rα is presented in trans to neighboring NK or CD8 T cells expressing only the IL-15RβγC receptor. As a co-stimulatory event occurring at the immunological synapse, IL-15 transpresentation now appears to be a dominant mechanism for IL-15 action in vivo and appears to play a major role in tumor immunosurveillance (Waldmann, T A, 2006, Nature Rev. Immunol. 6:595-601). Soluble IL-2Ra subunits, inducing isoforms containing a deletion of exon3 and the so-called “sushi” domain at the N terminus, have been shown to bear most of the structural elements responsible for cytokine binding. Whereas IL-2Rα alone is a low affinity receptor for IL-2 (Kd_10 nM), IL-15Rα binds IL-15 with high affinity (Kd_100 pM). Thus soluble IL-2Rα and IL-15 are able to form stable heterodimeric complexes in solution and these complexes are capable of modulating (i.e. either stimulating or blocking) immune responses via the intermediate or high affinity IL-15R complex (Mortier et al. 2006. J Biol Chem 281: 1612-1619; Stoklasek et al. 2006. J Immunol 177: 6072-6080; Rubinstein et al. 2006. Proc Natl Acad Sci USA 103: 9166-9171).
Given the known effects of IL-15 on the immune system, a number of IL-15-based approaches have been explored to manipulate the immune system for the hosts benefit. While IL-15 administration has been employed to bolster immune responses or augment immune system reconstitution, blockade of IL-15 activity can inhibit autoimmune and other undesirable immune responses (Waldmann, T A, 2006, Nature Rev. Immunol. 6:595-601). In fact, one of the limitations with systemic IL-15 treatment is the possible induction of autoimmune disease. Other limitations include the difficulty in produce this cytokine in standard mammalian cell expression systems as well as its very short half-life in vivo. Therefore, there is a need to generate a suitable immunostimulatory therapeutic form of IL-15 that displays a longer in vivo half-life, increased activity binding to immune cells, or enhanced bioactivity. Additionally there is a need for effective IL-15 antagonists. Ideally it would be desirable that such molecules be selectively targeted to the disease site to avoid unwanted systemic toxicities and provide a more effective therapeutic benefit.